Diffusive‐Convection Staircase Merger Events Mediated by Subsurface Eddies in the Canada Basin.

Diffusive‐convection (DC) staircases, characterized by interleaving well‐mixed layers and high‐gradient interfaces of thermohaline properties, are commonly observed in the Canada Basin, Arctic. Previous studies suggested a quasi‐steady state of the thermohaline structures of these staircases. In this work, we employ mooring and Ice‐Tethered Profiler (ITP) data to show emergent merger events of staircases mediated by subsurface eddies with warm cores located at 300–500 m depths in 2005, 2007 and 2009. Two types of merger events, the B‐merger (resulting from strengthening of stronger interfaces at the expense of weaker ones) and the H‐merger (produced by vertical drift and collision of staircase interfaces), are documented during the passage of subsurface eddies. The merging staircases are located at 320–450 m depths, with layers of up to 70 m thick and high‐gradient interfaces thinner than 5 m. Mooring measurements reveal the emergence of staircase mergers associated with a background Richardson number dropping from a typical magnitude of O(100) $\mathcal{O}(100)$–O(10) $\mathcal{O}(10)$ to O(0.1) $\mathcal{O}(0.1)$, indicating the persistence of DC staircases in the presence of strong shears. In parallel, ITP data suggest the formation of B‐mergers (H‐mergers) to be favored within the eddy core (flank). The vertical heat fluxes associated with the B‐merger (H‐merger) staircases are estimated to be ∼3 W/m2 (∼1.5 W/m2), which substantially exceed the typical staircase‐driven heat fluxes of O(0.1) $\mathcal{O}(0.1)$ W/m2. These observations imply the significance of subsurface eddies in shaping the halocline structures and thus the vertical heat fluxes in the Arctic Ocean, particularly in a warming climate. Plain Language Summary: In the Arctic Ocean, warm waters derived from the Atlantic Ocean lie beneath surface cold waters, and store sufficient heat capable of melting the Arctic ice cover. Residing between the surface cold and subsurface warm waters are stacked layers, commonly referred to as staircases. These staircases regulate the rate at which heat from underneath may reach the surface and melt the sea ice. This study employs field measurements in the Arctic Ocean to show that staircases can be substantially re‐structured to enhance the upward heat transfer if turbulent spinning bodies of waters named eddies are present at 300–500 m depths. Further calculations show that the existence of up to three such eddies may elevate the overall heat transfer of the western Arctic by up to ∼10%. In‐depth analyses on these processes are thus warranted, particularly in a warming climate that can increasingly "heat up" the subsurface Arctic Ocean. Key Points: Diffusive‐convection staircases are observed to persist and merge under strong vertical shearsBoth B‐mergers and H‐mergers are observed in individual subsurface eddiesVertical heat fluxes across merged staircases in subsurface eddies grow by one order of magnitude [ABSTRACT FROM AUTHOR]